The railroad industry uses assemblies of railroad switch fittings for railroad tracks that allow trains to divert their path and which are arranged at the beginning of the railroad switch; they are also known as rail devices whose purpose is to properly split or cross railroad tracks, even when they take different types of shapes. Rail road switches derive from the following key devices: the switch, which allows the crossing of vehicles from one track to another one; and the mid-rail, which makes the connection between the two switches.
At the switch, the axes of both tracks meet obliquely, and these axes cut each other at the mid-rail. To separate or cross the railroad tracks, two pieces of equipment are used; they are called railroad track switches and crossings, respectively.
When two railroad tracks cut each other, this can occur in an oblique or perpendicular way, thus resulting in two kinds of completely different devices: the oblique mid-rail and the rectangular mid-rail, where the former is found much more frequently. The oblique mid-rail can also be straight or curved, depending on the configuration of the crossing tracks.
The basic elements of a railroad device are mainly the switch and the crossing point. The railroad switch is made up by a pair of switching needles with accessories, a frog, a pair of guardrails and a set of switching railroad ties.
A frog is a crossing, a special building item for railroad tracks, that allows the crossing of two railroad tracks. They are assigned a number and a type; the number is the distance ratio of the intersection of two mid-rails to the width or distance between the squared lines at such distance. The frog number determines the frog angle, the degree of the diversion and the needle curvature, or the distance from the switching point to the frog spot.
In regards to the railroad switches, they make up a part of the switch that allows the separation of railroad tracks. Due to their extension, they are frequently called “needles”.
The devices more widely used are the simple switches, in which only one railroad, the diverted one, becomes separated from the general railroad, also called the main railroad or direct railroad.
The first switches built were made of needles or mobile railroad tracks and were articulated at the joint, on the diverted railroad, through a loose joint; this way, they could be arranged as an extension of any of the diverging railroad tracks. Obviously, with this arrangement, any vehicles taking the tracks not set with the needles will inevitably derail.
The railroad devices include, on the inside of the two continuous outside rails, two mobile pieces around the ends of the crossing side, and are conveniently placed to progressively adapt to such rails. These pieces, called needles, have a sharpened end due to the diversion at its adaptation point to the outside rails, called counter needles; the other end, where the rotation takes place, is called the heel.
At the common switches, both needles move concurrently tied by two or more braces, of which the closest to the end is joined to the maneuvering bar that runs under the railroads; these braces also serve to keep the adequate separation between the needles, that way when one of them is set against its counter needle, the other one leaves a clearance for the wheels' tabs. If this clearance is not achieved, and both needles are simultaneously against, or separated from, its corresponding counter needles, the derailing of the wheels will occur; in the first case, due to the narrowing of the railroad track and both wheels rolling over both needles; in the second case, due to the widening of the railroad track and both wheels from one axis rolling over the outside rails.
A switch can be molded on the end or on the heel. In the first case, a train coming from the common track will take one of the needles on its end and will follow the direction imposed by the needle; in the second case, the train will take the needles on their heels and will center the open track if the switch has been properly performed for the track carrying the train. If, by mistake, the switching maneuver is closed for this track, the wheels' tabs will lodge between the needle and the counter needle and will force their separation, thus breaking the braces that kept the normal separation between both needles. This situation is referred to as a heeled switch.
Regarding the crossings, this is the spot where the superimposition of the paths, taken by wheels whose tabs are on opposite sides, takes place. A discontinuity is forced upon both rolling paths for the wheels' tabs, between the end of the crossing point and the ends of the railroad tracks being cut off.
To restore the continuity of the rolling path, extension devices for such tracks are available. After being layered in such a way to form a path parallel to the opposite track, these tracks support the wheels by their border when their mid section is over the void, up until they fall on the corresponding track. Such extensions are called rabbit legs, and besides performing the aforementioned function, they also prevent the wheels of a vehicle moving on the arrow's direction to trip over the end of the cut off railroad track.
The spot where the other ends of the tracks meet is called the crossing heart point, where we can distinguish between the mathematical end from the real end that is somewhat behind the former due to the building pattern and to prevent its rapid wear. The end of the crossing piece is called the heel.
The crossing also includes the counter rails placed in front of the crossing angle along the outside railroad tracks, and whose purpose is to retain the outer wheels of vehicles traveling on the opposite direction to the arrows, thus preventing any swinging movement that would eject the inside wheels towards the crossing heart point, which besides causing its wear down, could also lead to the inside wheels taking the wrong path and causing the derailment.
Both, the rabbit legs and the counter rails widen slightly on their ends in order to avoid a hit from the wheels passing over and provide a smooth transition.
Since the wheel has a conical section, the wheel will descend while rolling over the rabbit leg and will then meet the heart's real end, unless the heart is lower than the corresponding point over the rabbit leg, which can be achieved by either progressively elevating the rabbit leg and keeping the heart in a horizontal plane, or by reducing the heart's elevation at least 5 mm below the level of the horizontal rabbit leg, which is the most common scenario. However, these circumstances, applicable to the new wheels, are modified when the wheels start wearing out, approaching a cylindrical shape, and at that stage, a more favorable scenario would be to have the rabbit leg and the heart's end meet at the same level. At the crossing points, a mixed system is generally used, which consists of slightly lowering the heart's end and giving the rabbit leg a trapezoidal or road hump profile, elevating it over a section of its length to then bring it back to a horizontal plane to the heart's end. Nevertheless, this procedure will not prevent some wheels from going higher or lower when going over the crossing point, depending on their wear.
The crossing points are generally arranged in a straight line, even when they have to alternate along curve railroad tracks, and they also tend to be symmetrical in regards to the angle's bisector, which allows them to be used on either direction.
The arrangement of the crossing points can be carried out in three different ways: by crossing the railroad tracks, with a special steel end and a crossing with molded steel.
The length of the counter rails is generally between 3.3 and 5.5 feet; they are placed in such a way that their middle section corresponds to the crossing void. The counter rails are generally built with ordinary rails that keep the necessary distance from the rolling path through the use of blocks, and special angular profiles are sometimes used in order to safeguard the structure's integrity and the invariability of the path.
Currently, the counter rail assemblies are used to hold and guide the train's wheels while rolling over the frog, where the counter rail supports and guides the wheel's tab.
The current assemblies and counter rail systems consist of a rail, where the train's wheels travel, and an adjacent counter rail. Between the rail and counter rail there is a separating seal, that just as the rail and counter rail, contains two transversal holes where the fixing devices go through to hold them tightly in place. This separating seal generates a separation between the rail and counter rail where the wheel's tab should go through. The separation must measure 4.76 cm (17/8 of an inch), or other measurement depending on the regulations.
Since the railroad tracks are made up of a profile that defines a head, a middle section and a skid, when the counter rail is incorporated along the rail where the train's wheels go through, a part of the counter rail's inside skid must be cut off in order to allow the approximation towards the rail and its attachment to the support plates. The continuous travel of the wheels, leads to the wear down of the counter rail's inside header surface caused by the wheel's tab, thus increasing the separation where the tab passes through. To fix this problem, the counter rail must be discarded and replaced.
To counteract this situation, an innovative counter rail system has been developed to switch railroad tracks, thus solving the aforementioned inconveniences.